1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus including an image bearing member and a developing device.
2. Description of Related Art
An electrophotographic image forming apparatus irradiates a circumferential surface of an image bearing member (photosensitive drum) with light based on image information which is obtained by reading an image of a document and transmitted from an external instrument such as a computer. Accordingly, the electrophotographic image forming apparatus forms an electrostatic latent image, supplies the electrostatic latent image with toner to form a toner image, and thereafter, transfers the toner image to a sheet. The sheet subjected to such transfer processing is then subjected to fixing processing of the toner image, and is thereafter delivered to the outside.
By the way, in recent years, in the image forming apparatus, an apparatus structure thereof has become complicated as color printing has been increased and a processing speed has been enhanced, and in addition, high speed rotation of a toner stirring member in a developing device has been forced in order to cope with such enhancement of the processing speed. In this case, a pressure inside the developing device is prone to be a positive pressure higher than the atmospheric pressure. When the pressure inside the developing device becomes the positive pressure, in the event where the toner is supplied from the inside of the developing device to the photosensitive drum, a part of the toner becomes floating toner and is discharged from a toner supply port to the photosensitive drum, to thereby contaminate an inside of an apparatus main unit of the image forming apparatus. In addition, necessary toner becomes less likely to be supplied to the electrostatic latent image. As a result, such a malfunction becomes prone to occur, that there occurs a so-called toner lack phenomenon in which the toner is not supplied to a portion on the circumferential surface of the photosensitive drum, on which the toner should be stacked.
In this connection, a variety of developing devices are known, which are improved so as to become capable of preventing leakage of the toner. For example, in a developing device that we will refer to as Related art 1, in a casing inside thereof, there are provided a stirring roller and a developing roller, the developing roller being arranged on a downstream side of the stirring roller and supplying the toner to the circumferential surface of the photosensitive drum. The stirring member is arranged at a little upper position than a bottom plate of the casing, and the developing roller is arranged at an obliquely upper position of the stirring member.
Then, at an upper position of the stirring member, a partition plate is obliquely provided, which guides the toner so that the toner can flow toward a circumferential surface of the developing roller. Owing to the presence of this partition plate, an inside of the casing is divided up and down into halves. The toner above the partition plate passes through a gap between a lower end edge portion of the partition plate concerned and the bottom plate of the casing, and is supplied to a space below the partition plate. The toner supplied here is subsequently drawn up onto the circumferential surface of the developing roller by drive/rotation of the stirring roller, and is supplied from the circumferential surface of the developing roller to the circumferential surface of the photosensitive drum.
In the developing device as described above, a vent hole is drilled in the partition plate, and thus it is made possible to take out air in the space below the partition plate, of which pressure has become the positive pressure by the rotation of the stirring roller, through the vent hole concerned to a space above the partition plate. In such a way, a pressure in such a space where the stirring roller is provided is decreased. Accordingly, air containing the floating toner is stopped being discharged to the outside from a toner discharge port. As a result, the leakage of the floating toner from the toner discharge port can be prevented.
Meanwhile, it is known that, in a developing device that we will refer to as Related art 2, air in a gap between a toner carrying member and a covering member that covers a part of an outer circumferential surface of the toner carrying member is forcibly sucked, whereby scattering of the toner and heat generation of the developing device are suppressed even if a speed of a developing process is enhanced.
By the way, in the developing device of Related art 1, the air containing the floating toner, which is present in the space below the partition plate in the casing, passes through the vent hole drilled in the partition plate, and is caused to flow toward the space above the partition plate, and in this case, a flowing direction of the air is substantially orthogonal to a transporting direction of the toner fed onto the photosensitive drum by the rotation of the developing roller.
Therefore, in many cases, strength of an airflow attempting to go toward the vent hole is not so strong as to cancel a flow of air attempting to accompany the rotation of the developing roller. As a result, there is a problem in that the air containing the floating toner becomes prone to flow toward the circumferential surface of the photosensitive drum by the rotation of the developing roller, resulting in leakage of the air concerned from a toner supply opening of the casing opposed to the same circumferential surface.
Further, in the developing device, in some case, the toner that has scattered is deposited on the periphery of an ear cutting blade. In the case where the deposited toner is aggregated and adhered onto the developing roller, there is also an apprehension that such a toner lack may occur to cause a malfunction of the image.
Accordingly, for example, as illustrated in FIG. 16, in the developing device, it is conceived to provide an air inflow port, which is for taking in air from the outside of the developing device, in a wall portion of a developing container, which is opposed to the developing roller. FIG. 16 is a schematic sectional side view illustrating a structure of the periphery of the developing device in which the air inflow port is provided. As illustrated in FIG. 16, in a developing container 20 of a developing device 3a, there are provided a stirring-transport screw 25a, a supplying-transport screw 25b, a magnetic roller 30, and a developing roller 31.
A developer containing toner and a carrier, which is stirred and transported by the stirring-transport screw 25a, is supplied to the magnetic roller 30, which rotates clockwise in FIG. 16, by the supplying-transport screw 25b. Then, the developer forms a magnetic brush on the magnetic roller 30. Only the toner in the magnetic brush on the magnetic roller 30 is carried on the developing roller 31 rotating clockwise in FIG. 16, and the toner is supplied to a photosensitive drum 1a from the developing roller 31.
In a space on a left side of the magnetic roller 30 and the developing roller 31 and also above an ear cutting blade 35, airflows as illustrated by broken lines are individually generated by the clockwise rotation of the magnetic roller 30 and the developing roller 31, the two airflows illustrated by the broken lines join together, and as a whole, an airflow going upward as illustrated by a solid line is generated. Therefore, even in the case where the floating toner is present in the vicinity of the ear cutting blade 35, the toner can be prevented from being deposited on the ear cutting blade 35 by bearing the floating toner on this airflow.
Therefore, in the developing container 20, a wall portion 20c protruding inward is provided above the ear cutting blade 35 in FIG. 16, and two air inflow ports 51a and 51b are provided in the wall portion 20c. Thus, an airflow flowing inside the developing container 20 from outside flows into the vicinity of the ear cutting blade 35, in which a pressure becomes negative by the generation of the airflow illustrated by the solid line.
By such inflow of the airflow, it becomes possible to decrease a pressure of the portion, which has become positive by the rotation of the magnetic roller 30 and the developing roller 31. Further, the airflow that has flowed in connects to the airflow going upward, and hence the floating toner in the vicinity of the ear cutting blade 35 is borne on the airflow going upward, thus making it possible to prevent the toner from being deposited on the ear cutting blade 35.
However, even in the case where the air inflow ports are provided as described above, in the case where the processing speed of the apparatus main unit is enhanced, and a floating amount of the toner is large, it becomes difficult to sufficiently bear the floating toner on the airflow generated by the rotation of the magnetic roller and the developing roller, and it becomes difficult to sufficiently prevent the deposition of the toner.
Further, in the developing device of Related art 2, the toner supplied from the stirring screw onto the developing roller is supplied to the photosensitive drum, and such a technology is not used, in which the toner is carried on the developing roller from the stirring screw through the magnetic roller, and the toner is supplied to the photosensitive drum. Further, depending on whether or not the magnetic roller is present, the portion in the developing device, in which the positive pressure is generated, is changed as well as a magnitude of the positive pressure.